The invention relates to oxygen-free copper, produced without the use of deoxidizers, rather than deoxidized copper. Conventional oxygen-free coppers are known to typically contain less than one part per million by weight of manganese as an impurity, and also contain other impurities. For example, the ASTM Standard Specification B 170-72 for Oxygen-Free Electrolytic Copper Wire Bars, Billets and Cakes provides for Grade 1 (minimum electrical conductivity of 101% I.A.C.S.) that the total impurities present be no more than 100 parts per million by weight, that the oxygen impurity present be no more than 10 parts per million by weight, that the sulphur impurity present be no more than 18 parts per million by weight, and that the total of all arsenic, antimony, bismuth, selenium, tellurium, tin and manganese impurities present be no more than 40 parts per million by weight. It is generally desirable to control and reduce impurities in oxygen-free copper.
It is well known in the art that to obtain desired properties in wrought oxygen-free copper, the metal should be properly worked and annealed. Also as known, annealing involves the process of recovery, recrystallization and grain growth, with the grain size of an annealed copper depending, among other things, on the time and temperature of the annealing operations and the nature and amount of cold working to which the metal has been subjected. Difficulties arise during such operations, however. After heating conventional oxygen-free coppers at temperatures from 750.degree. to 1,400.degree. F as in most commercial annealing operations, or at temperatures from 1,400.degree. to 1,900.degree. F as in brazing, a mixture of widely different grain sizes (i.e., an abnormal grain structure) usually develops, with larger grains often being of the order of ten times larger in diameter than the smaller grains. The art has considered such a grain structure to be highly undesirable, and the inventors here believe that it often leads to roughened, or "orange peel", surfaces of the copper. The abnormal grain structure further is thought by the inventors here to set up abnormal stresses during cold working or during use, and uneven flow during cold forming. A brittle form of cracking also occurs in oxygen-free copper even when the sulphur content is less than 18 parts per million by weight.